Segmented copolymers of ethylene oxide and styrene, acrylonitrile, or methyl methacrylate are known in the art. U.S. Pat. No. 3,257,476 teaches block polymers of polyvinyl monomers, for example, styrene and polypropylene oxide. Specifically, that patent teaches the preparation of such block copolymers by the reaction of hydroperoxide compounds with polymeric compounds containing isocyanate groups to create polymeric peroxy carbamates. These peroxy carbamates are then used to catalyze the free radical polymerization of ethylenically unsaturated monomers. While this approach can be employed to prepare block copolymers, it has the disadvantage that the ethylenically unsaturated monomer can also homopolymerize. Consequently, the resulting polymers are invariably accompanied by substantial amounts of polyvinyl homopolymer (this is pointed out in U.S. Pat. No. 3,257,476).
Another description of block copolymers of ethylene oxide and styrene, methyl methacrylate and acrylonitrile has been reported by Brooks and Daffin (Polymer Preprints, Vol. 10, No. 2, September 1969, p. 1174). These workers utilized the approach of U.S. Pat. No. 3,257,476 to prepare block polymers for examination as membranes. They observed that (1) vinyl homopolymer was necessarily obtained as a contaminant, (2) the resulting polymers possessed very poor strength properties, and (3) were so troublesome to work with that those investigators were motivated to examine other systems.
The present invention differs from that of U.S. Pat. No. 3,257,476 in that the block copolymers prepared are substantially free of polyvinyl homopolymer. The importance of preparing block copolymers free of homopolymers of either block will be shown subsequently in this invention. It has been found surprisingly that physical properties such as clarity, tensile strength, elongation, and flexibility, and general mechanical properties are dramatically superior in those block polymers which are substantially free of homopolymer contamination. Obviously this finding is of tremendous importance from an applications viewpoint.
U.S. Pat. No. 3,050,511 teaches block copolymers of polystyrene and ethylene, propylene, or 1,2-butylene oxide. The compositions taught by that patent differ from the instant compositions in that those patented compositions are AB or BAB block polymers wherein A represents the hydrophobic portion, i.e. polystyrene, and B represents the hydrophilic portion, i.e. polyethylene oxide. The compositions of the instant invention are those represented by graft copolymers having the general formula ##STR3## and block copolymers having the general formula xB--[AB].sub.n --yA wherein n is an integer .gtoreq.1, m is &gt;1, x and y are 0 or 1, and y is 1 when n is 1. It is only the compositions of this invention which provide a unique combination of properties not provided by two block systems, or the three block systems in which the center block is the hydrophobic block.
Finally, a recent publication (Shimura and Lin, J. Poly. Sci., Part A-1, 8, 2171 (1970)) discloses that block copolymers of polystyrene and polyethylene oxide can be prepared via a process similar to one described in the present invention. They prepared [AB].sub.n block polymers wherein the A blocks or polystyrene blocks varied from 2,250 to 6,200 in molecular weight, and the B blocks or poly(ethylene oxide) varied from 404 to 5,650. They report that only soft, elastic products could be achieved, and, indeed, for block polymers where the polyethylene oxide (or polyethylene glycol) blocks were longest (5,650 molecular weight) the polymer was too soft to be measured at ordinary temperature ranges.
The products of the instant invention differ dramatically from those above for reasons that will be discussed subsequently in some detail. The products of this invention are hard, rigid multiphase plastics. Clearly these properties are unexpected in view of the prior art.